Configuring Dhcp Relay; Overview; Dhcp Option 82 - HP ProCurve 6200yl Multicast And Routing Manual

IP Routing Features

Configuring DHCP Relay

Configuring DHCP Relay

Overview

The Dynamic Host Configuration Protocol (DHCP) is used for configuring
hosts with IP address and other configuration parameters without human
intervention. The protocol is composed of three components: the DHCP client,
the DHCP server, and the DHCP relay agent. The DHCP client sends broadcast
request packets to the network, the DHCP servers respond with broadcast
packets that offer IP parameters, such as an IP address for the client. After
the client chooses the IP parameters, communication between the client and
server is by unicast packets.
The function of the DHCP relay agent is to forward the DHCP messages to
other subnets so that the DHCP server doesn't have to be on the same subnet
as the DHCP clients. The DHCP relay agent transfers the DHCP messages from
DHCP clients located on a subnet without DHCP server, to other subnets. It
also relays answers from DHCP servers to DHCP clients.

DHCP Option 82

Introduction
Option 82 is called the Relay Agent Information option and is inserted by the
DHCP relay agent when forwarding client-originated DHCP packets to a
DHCP server. Servers recognizing the Relay Agent Information option may
use the information to implement IP address or other parameter assignment
policies. The DHCP Server echoes the option back verbatim to the relay agent
in server-to-client replies, and the relay agent strips the option before
forwarding the reply to the client.
The "Relay Agent Information" option is organized as a single DHCP option
that contains one or more "sub-options" that convey information known by
the relay agent. The initial sub-options are defined for a relay agent that is co­
located in a public circuit access unit. These include a "circuit ID" for the
incoming circuit, and a "remote ID" which provides a trusted identifier for the
remote high-speed modem.
The routing switch can operate as a DHCP relay agent to enable communica­
tion between a client and a DHCP server on a different subnet. Without Option
82, DHCP operation modifies client IP address request packets to the extent
needed to forward the packets to a DHCP server. Option 82 enhances this
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